Telecommunications line parameter estimation

ABSTRACT

A fault location system for a telecommunications network including a local exchange or switch, a node such as a primary connection point and a plurality of line terminations to the customer includes probability calculation to establish the location of a fault measuring from the exchange. Historical non-faulty reference values of capacitance are compiled for each line passing through the node providing a measure of the distance of each line. A lowest valid capacitance value provides an estimate of the reference capacitance between the exchange and the node. Using various parameters, probability tables are compiled of historic fault values and a new fault is compared against these tables to establish the probability of it being a fault at the node or elsewhere. As a result the reference capacitance can be obtained without the need for an engineer at the node, and the likely location of a fault can be pinpointed with greater accuracy.

This application is the U.S. national phase of international applicationPCT/GB01/00974 filed 6 Mar. 2001 which designated the U.S.

BACKGROUND

1. Technical Field

The invention relates to a line parameter capacitance estimation methodand system and a fault management system incorporating such a system.

2. Related Art

A conventional public telecommunications network comprises a relativelysmall number of interconnected main switches and a larger number oflocal switches, each of which is connected to one or two main switches.The local switches are connected to the terminating lines of the networkand the far ends of these lines are connected to terminal equipment suchas telephone instruments provided for users of the network.

The network formed from the main switches and local switches is known asthe core network, while the network formed from the terminating lines isknown variously as an access network or a local loop. Some terminatinglines are connected to a remote concentrator, which may or may not haveswitching capabilities. The remote concentrator is then connected to alocal switch. The term local switch when used herein covers both localswitches and remote concentrators.

In a conventional access network, each terminating line is formed from apair of copper wires. Typically, each copper wire passes through aseries of nodes between the local switch and terminal equipment.Examples of such nodes are primary cross-connection points (PCP),secondary cross-connection points (SCP), distribution points (DP) andjunctions.

Terminating lines are prone to faults such as dis-connections, shortcircuits between the two wires of the pair of wires, and short circuitsbetween one of the wires and earth. Causes of such faults includeingress of water into a node and also physical damage to a node.

Disconnection faults occur frequently; 30-40% of all logged faults aredisconnection faults and 25% are at the PCP making these the most commonkind of disconnection fault. According to known systems, the engineer issent to the PCP to establish whether the fault is there or in thevicinity which may be as little as a street or two away. However it isdesired to be able to identify within a reasonable level of accuracywhether the fault is actually at the PCP or, for example, with thecustomer, in order to reduce the engineer time spent on locating thefault.

Local switches are provided with line testing apparatus which may beused to test its terminating lines. When a customer reports a fault on aterminating line, the line may then be tested to identify the faultcondition.

The process of locating, evaluating and repairing a fault may involve acomparison between the fault value with a reference capacitance which isthe capacitance of a line under normal conditions between a node, suchas a PCP, and the local switch.

Conventionally, it has been necessary for a person to travel to a nodeto test the node. Simultaneously, it is necessary to test the line fromthe local switch. This procedure suffers from several disadvantages.Firstly, as it is necessary to send a person to the node to measure acapacitance reference value, two people are needed to complete themeasurements—one at the node and a person at the local switch. Theprocedure itself is labour intensive, and time consuming. Also, becauseit is also necessary to test several nodes before the correct one isfound, the problems of cost and time are exacerbated. Known systems aredescribed in published patent applications EP 0862828 and EP0938800.

According to one known system, the location of the fault is thenestimated using an expert system assigning scores based on a range ofparameters. This system requires complex processing of data on site. Inaddition, where there is more than one line running to the PCP, orcabinet, a single reference value may not be representative if there isre-routing or if a line between the switch and exchange follows analternative route where there are multiple lines.

BRIEF SUMMARY

According to the invention there is provided a line parameter estimationmethod for a telecommunications network including a switch and aplurality of terminating lines extending from the switch through a node;comprising the steps of measuring a parameter of each terminating line;compiling line test data from the measured capacitances; calculating avalid limit value from the compiled line test data; and determining anestimated switch to node line parameter as the valid limit value.

This method offers several advantages over the prior art. It eliminatesthe need for an engineer to travel to one or more nodes, such as PCP's,to obtain a reference value for, for instance, the node to switchcapacitance. It reduces the manpower required to obtain such areference—previously a person was additionally required at the switchsimultaneously to the person at the node. The time and expenditureassociated with such a procedure is significantly reduced.

Also, the previous method was prone to human error. According to thepresent invention, the estimation method can be automatic and thereforeis not prone to such errors. Further, the estimation can always beavailable, and does not require time for a person to travel to a nodeand make a measurement before it is available. In one embodiment, themeasurement of capacitance can conveniently be made overnight, therebyminimizing disruption to normal services.

The term “switch” includes both local switches and remote concentrators,and the term node includes PCP's, SCP's, distribution points andjunctions.

Preferably, parameter includes capacitance.

The node may be a primary cross-connection point, and preferably theterminating lines have a capacitance substantially dependent upon alength of the terminating line.

Preferably, the set of line test data excludes data for which themeasurements of capacitance are substantially independent of a length ofthe terminating line. As a result, the set of line test data excludesline test data from Digital Access Carrier Systems, Integrated ServiceDigital Networks, and WB 900 analogue pair gain systems; or lines ofsimilar characteristics.

The set of line test data may exclude line test data from lines forwhich a corresponding resistance associated with the lines is less thana predetermined value. This offers the advantage of avoiding anyproblems which may be associated with the accuracy of capacitance valuesobtained from lines tested in the resistance of the lines falls belowsome critical level.

Preferably the compiled line test data includes only healthy measurecapacitance value measurements from lines for which a correspondingmeasured resistance associated with the line is less than apredetermined value. This minimizes the biasing of the results owing topossible rogue capacitance readings.

According to the present invention, there is also provided a lineparameter estimation system for a telecommunications network including aswitch and a plurality of terminating lines extending from the switchthrough a node; the system comprising measuring means to measure theparameter of each terminating line; means to compile line test data fromthe measured parameters; calculating means to calculate a valid limitvalue from compiled line test data; and estimating means to determine aswitch to node line parameter as the valid limit value.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be put into practice in several ways. Aspecific embodiment will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram of an access network and an associated localswitch which form part of a telecommunications network according to theprior art;

FIGS. 2A and 2B are histograms of healthy capacitances for lines througha node constructed according to the method of the present invention;

FIG. 3 is a flow chart illustrating the method of obtaining a minimumdistribution of healthy reference values for a node according to thepresent invention.

FIGS. 4 a and 4 b are a diagrammatic representations of atelecommunications network.

FIG. 5A shows two sample values on a PCP reference histogram;

FIG. 5B shows multiple bin values on a PCP historic reference histogram;

FIG. 6A is a flow diagram showing the population of a histogram for aPCP reference table;

FIG. 6B is a flow diagram showing the population of a historic PCPreference table;

FIG. 7 is a flow diagram showing the calculation of probability of a PCPfault;

FIG. 8 is a flow diagram for the algorithm for populating a PCPreference table;

FIG. 9 is a flow diagram showing the algorithm for populating a PCPhistoric reference table; and

FIG. 10 is a flow diagram showing calculation of a relevant values forprobability assessment on the PCP reference table.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The fault location system and method of the present invention comprisestwo basic operations. The system provides an arrangement capable offinding the likely location of a fault in a telecommunications systemeffectively by comparing a test measurement on a line with a referencevalue and deriving from that comparison the likely location of thefault. Accordingly, the two operations are respectively obtaining one ormore reference values to form the basis of the comparison, and carryingout the comparison itself.

FIG. 1 illustrates an access network 12 of a conventionaltelecommunications network connected to a local switch 10. The localswitch 10 and the access network 12 form part of a telecommunicationsnetwork.

The local switch 10 is connected to the terminating line of the accessnetwork 12. Typically, a local switch is connected to several thousandterminating lines. Each terminating line passes through several nodesbefore reaching its respective terminal equipment. These nodes compriseprimary cross-connection points (PCP), secondary cross-connectionpoints(SCP), distribution points (DP) and junctions.

Each terminating line is typically formed from a pair of copper wires.The copper wires leave the local switch 10 in the form of one or morecables. One of these cables is shown in FIG. 1 and is indicated byreference numeral 14. The far end of cable 14 from switch 10 isconnected to a PCP 16 which may be housed in a street cabinet orunderground junction box. From the PCP 16 the terminating lines branchout as cables in several directions. For simplicity, in FIG. 1 there areshown only three cables 18, 20 and 22. The far end of cable 18 isconnected to a joint 19. The joint 19 is connected by cable 21 to a SCP24. The far ends of cable 20 and 22 are connected to, respectively, SCP26 and 28. For reasons of simplicity, the continuation of theterminating lines beyond SCP 24 and 26 is not shown. The SCP 24, 26 and28 are housed in junction boxes which may be located above or belowground.

For the SCP 28, the terminating lines branch out again in severaldirections in the form of cables. By way of illustration, FIG. 1 showscables 40, 42 and 44 leaving the SCP 28. Cables 40 and 44 are connected,respectively, to joints 46 and 48. Joints 46 and 48 are connected,respectively, to cables 50 and 52. the far ends of which are connectedto distribution points 54 and 56.

The far end of cable 42 is connected to joint 60. The joint 60 isconnected by cable 62 to a distribution point 64. For reasons ofsimplicity, the terminating lines beyond distribution points 54 and 56are not shown.

Distribution points are implemented as junction boxes which aretypically located on telephone poles or can be internal or underground.For each distribution point, the terminating lines branch out as singlecopper cable pairs to where terminal equipment provided for a user ofthe network is located. By way of illustration, FIG. 1 shows two singlecopper cable pairs 70, 72 leaving the distribution point 64. The farends of cables 70 and 72 are connected, respectively, to terminalequipment 74, 76. The terminal equipment take various forms, forexample, a public telephone located in a public telephone box, atelephone instrument located in a house or office, or a fax machine orcomputer located in a customer's premises.

In the example shown in FIG. 1, each of the joints 19, 46, 48 and 60 areused to connect the cables together. Joints may also be used to connecttwo or more smaller cables to a large cable.

The cable 14 is housed in a duct in which the air is kept at a pressureabove ambient pressure.

In each terminating line, the two wires of each pair are designated asthe A leg and B leg. At the local switch 10, in order to supply currentto the line, a bias voltage (sometimes termed a “battery” voltage) of 50V is applied between the A leg and B leg. In the terminal equipment, theA leg and B leg are connected by a capacitor, the presence of which maybe detected when the terminal equipment is not in use.

The terminating lines in the access network are prone to faults. Themain cause of these faults are ingress of water and physical damage tothe nodes through which the terminating lines pass between the localswitch 10 and terminal equipment. There are five main faults which occurdue to causes arising in the nodes. These faults are disconnection,short circuit, faulty battery voltage, earthing faults and lowinsulation resistance. A disconnection arises where a terminating lineis interrupted between the local switch and the terminal equipment. Ashort circuit arises where the A leg and B leg of a line are connectedtogether. A faulty battery voltage arises where the A leg or the B legof a terminating line has a short circuit connection to one of the wiresof another line. An earthing fault arises when the A leg or the B leg isconnected to earth. Low insulation resistance arises where theresistance in the cable between the A leg or the B leg or between one ofthe legs and earth is below an acceptable value.

When a line is tested, the leg to earth capacitance returned isgenerally directly proportional to the length of that line. Hence, if adisconnection fault occurs on a leg, a reduction in the leg to earthcapacitance for that leg occurs compared to a non-faulty or “healthy”measurement.

The line testing equipment itself is well known to the skilled personand is available commercially. For example, a suitable line tester for aswitch is available from Porta Systems of Coventry, England. Similarly,measurement systems based on capacitance values from line tests are wellknown, for example the Vanderhoff measurement system and the Teradyneline tester from Teradyne Limited of Western Centre, Western Gate,Bracknell RG12 1RW, England.

Turning now to the fault location method and system of the presentinvention, it is first of all necessary to obtain a reference valueparameter against which values from line tests can be compared asdiscussed in more detail below. In the present instance, the referencevalue used is the healthy capacitance reference from the exchange (localswitch 10) to the PCP. It will be appreciated that this is not a fixedvalue for all switches and PCP's but is dependent on physical attributesof the network.

There can be several different cables running from the switch to a node,such as a single PCP. These e-side (exchange side) cables do notnecessarily follow the same physical route from the switch to the PCP.Hence, the physical length of the e-side cables can differ resulting insome lines with similar d-side (distribution side) routing having verydifferent electrical characteristics.

Also, it is assumed that the lines in a particular cable can be routedanywhere on the d-side of a node, such as a PCP. it follows from thisassumption that an approximation of the switch to PCP capacitance forthe shortest e-side cable can be extracted from examining all of thehealthy capacitance references of the lines routed through the PCP.

One hypothetical but very direct way of obtaining a healthy linereference value would simply be to obtain a capacitance value for a lineto a telephone at the PCP, or to a line deliberately disconnected at thePCP and hence terminating there. Either of these approaches would, intheory, give the exact value of the capacitance of the line between theswitch and the PCP. However, these approaches give rise to variousproblems. First of all, such measurements would be based on ameasurement of one e-side line, i.e., a single pair on a single cable.This may not be representative of an appropriate reference value wheremultiple lines are routed from the switch of the PCP. In particular, thesingle measurement might not represent the majority of the linesavailable, furthermore, not all PCP's (or cabs) have cab phones andhence this approach could not be used universally. On the other hand,deliberately disconnecting a line at each PCP would be highly timeconsuming. As a result, a reference value measurement system that can becontrolled from the switch and which is representative of the physicalrealities of the network is requested.

The basic approach taken according to this aspect of the invention is toobtain healthy line references from line test data from all or arepresentative number of lines on the PCP. Of those values, based on therule adopted that line length is directly in proportion to capacitance,the lowest value of capacitance will correspond to those linesterminating nearest the PCP and hence will provide a close approximationof the PCP reference value. The preferred method is discussed in moredetail below with particular reference to identifying healthy linevalues, a representative sample of measurements and excluding any aftereffects.

According to one embodiment of the present invention, the first step inestimating the reference capacitance for a line between a node, here thePCP, and the local switch is to obtain healthy line references from linetest data for all lines, taken over a predetermined time, for example 10days. The measurements are preferably taken overnight. Some lines maynot necessarily be tested each day for the whole of the period.

In the first instance, the results are restricted to representative linetest data. All reports are assigned a code following standard procedure(a termination statement) and tests associated with faulty lines areignored. Other parameters are measured beyond the capacitance, one ofwhich is line resistance and tests associated with a resistance valueless than 1 MΩ are also disregarded. This minimum value is introducedbecause of possible inaccuracies of capacitance values obtained whenlines are tested when resistance of the lines falls below some criticallevel. In practice it may be possible to relax the level, for example,100 kΩ without impairing the accuracy of the system.

Occasionally, rogue capacitance readings occur which bias the results.In order to overcome this problem, the average and standard deviationsof the A and B leg to earth resistance values over this period arecalculated. Only lines with individual leg to earth standard deviationsof less than a predetermined value, in this embodiment 10 nf, areconsidered as potential healthy reference values.

Following this initial filter, the remaining lines are examined to seeif they balance, that is to establish that the leg to earth capacitancemeasurement of each leg of the pair are close enough together, in thepreferred embodiment, within 80% of each other. This balance requirementis necessary as the healthy reference value is a single value that isthe average of the A leg and B leg to earth capacitance values, asdiscussed below.

In summary, the healthy references for each line which are denoted Hbelow are determined from averages of the leg to earth capacitance from

-   -   a predetermined period of time, in this case 10 days    -   termination statement is such that the customer equipment or        master jack is detected    -   balanced, steady lines with resistance measurements >1 MΩ    -   lines with standard deviations of individual leg to earth        capacitance less than 10 nf

As will be discussed in more detail below in the specification inrelation to the second part of the operation, a healthy reference valueis set up and stored for as many lines as possible as subsequent faultdetection is based upon that value.

One further source of artifact originates from lines having specialcharacteristics such as digital DACS lines, WB900 and ISDN2. These givecapacitance readings that are independent of the length of the actualline. For example, in the example of DACS these lines return apredefined set of test results irrespective of the length of the line.As discussed in more detail below, a further exclusion relates to valuesreturned from lines having these special characteristics.

The healthy reference values retained are, in the preferred embodiment,presented in the form of a histogram as shown in FIGS. 2A and 2B. Asdiscussed above, it is assumed that the lines in a particular, cable canbe routed anywhere on a d-side of a PCP. It follows that anapproximation of the exchange to PCP capacitance with the shorteste-side cable can be extracted by examining all of the healthycapacitance reference lines routed through the PCP. Referring to FIG.2A, the lowest leg to earth capacitance value (disregarding artifacts)is selected as representative of a healthy PCP reference value.

The algorithms and resultant histograms are now discussed in more detailwith reference to FIG. 3 and Tables 1 to 6.

Table 1 lists exemplary user definable parameters and possible valuesfor the preparation of healthy line references.

The parameters are “global” in the sense that they are applicable to allregions.

TABLE 1 Possible Parameter Parameter of range to Suggested CodeDescription Units set parameter value P1_1 Minimum number — Greater than  10 of routing 0 to less than measurements P1_2 P1_2 Maximum number —P1_1 to 30   20 of routing measurements P1_3 Minimum healthy Ohms 0 to999999 999999 A leg to B leg resistance threshold P1_4 Minimum healthyOhms 0 to 999999 999999 A leg to Earth resistance threshold P1_5 Minimumhealthy Ohms 0 to 999999 999999 A leg to battery resistance thresholdP1_6 Minimum healthy Ohms 0 to 999999 999999 B leg to A leg resistancethreshold P1_7 Minimum healthy Ohms 0 to 999999 999999 B leg to Earthresistance threshold P1_8 Minimum healthy Ohms 0 to 999999 999999 B legto battery resistance threshold P1_9 Healthy Termination — List of IFVanderhoff Condition allowed system, THEN termination P1_12 = statementsEITHER 1 or 3 IF Teradyne system, THEN P1_12 = 7 P1_10 Minimum number —1 to P1_1    1 of healthy measurements P1_11 Balance condition — 0 to 1     0.8 P1_12 Steady condition Nano 0 to 00   10 Farads

Routing information for the area to be investigated is also required forthe purposes of associating a reference value with a line. Table 2 givesthe data fields needed for each line.

TABLE 2 Code Data Field Name Description L_CIRC CIRCUIT_ID Identifiesthe line L_TYPE DP_NED_TYPE Type or service (null to avoid DACS, etc)L_PCP PCP_EO_NUMBER Identification number for PCP for the line L-DISTDISTRICT_ID District identification code L-EXCH EXCHANGE_CODE Exchangeidentification code

A PCP is uniquely identified be three pieces of routing information,described in Table 2, L_DIST, L_EXCH and L_PCP.

Only lines with “known routing” are used. A line is said to have knownrouting when the following conditions are met:

-   L_CIRC has an entry;-   AND L_LINENO has not got an entry or is one;-   AND L_TYPE is not one of the excluded types of service, e.g. DACS;-   AND L_PCP has an entry;-   AND L_DIST has an entry;-   AND L_EXCH has an entry.

Additional local network routing information is given in Table 3necessitated as L_CIRC can have two components—a directory number and aline number.

TABLE 3 Code Data Field Name Description L_DIRNO DIRECTORYNUMBERDirectory number for the line L_LINENO LINENUMBER Number of the line onthe directory

The excluded types of service are lines that have certaincharacteristics (L_TYPE). As discussed above, an example is a DACS line(digital access carrier system), which gives a capacitance readingindependent of the length of the actual line. Other examples are WB 900(a model number of an analogue gain system) and an ISDN (integratedservice digital network).

To produce healthy line references, between P1_1 and P2_2 routingmeasurements taken on different days during a continuous P1_2 dayperiod, are collected for lines with known routing. The night routingdata fields are described in Table 4.

TABLE 4 Code Data Field Name Description R_DIRNO DIRECTORYNUMBERDirectory number for the line R_LINENO LINENUMBER Number of the line onthe directory number R_RESAB RESISTACEAB Resistance value, used to checkif line healthy, A-B leg R_RESAE RESISTANCEAE Resistance value, used tocheck if line healthy, A-Earth R_RESABAT RESISTANCEATOBATT Resistancevalue, used to check if line healthy, A-Battery R_RESBA RESISTANCEBAResistance value, used to check if line healthy, B-A leg R_RESBERESISTANCEBE Resistance value, used to check if line healthy, B-EarthR_RESBBAT RESISTANCEBTOBATT Resistance value, used to check if linehealthy, B-Battery R_CAPAE CAPACITANCEAE Capacitance, A leg to EarthR_CAPBE CAPACITANCEBE Capacitance, B leg to Earth R_TERM TERMSTATEMENTAFM termination statement, used to check if line is healthy

Line test data is considered as healthy data if it satisfies thefollowing requirements:

-   R_RESAB>P1_3-   AND R_RESAE>P1_4-   AND R_RESABAT>P1_5-   AND R_RESBA>P1_6-   AND R_RESBE>P1_7-   AND R_RESBBAT>P1_8-   AND R_TERM IN LIST P1_9

Accordingly, all resistance values (leg to earth, leg to battery/biasvoltage and to loop of each pair) are tested against the 1 MΩ thresholdvalue.

It will be seen that a final test is to establish that a “healthy line”as identified (R_TERM). The relevant value depends on the test system,either the Vanderhoff system, or the Teradyne test system.

Also, any line test data associated with a fault report were ignored.

In order to implement the step of assembling and implementing standarddeviation values for the capacitance for each of the lines with knownrouting, the number n of healthy data measurements is counted. If n isgreater than or equal to a minimum P1_10, then the following arecalculated:

${AV\_ CAPAE} = {\sum\limits_{i = 1}^{n}\frac{\left( {R\;\_\; C\; A\; P\; A\; E} \right)_{i}}{n}}$(The Average of the R_CAPAE measurements)

${S\; D\;\_\; C\; A\; P\; A\; E} = \sqrt{\sum\limits_{i = 1}^{n}\frac{\left. \left( \left( {\left( {R\;\_\; C\; A\; P\; A\; E} \right)_{i} - {A\; V\;\_\; C\; A\; P\; A\; E}} \right) \right) \right)^{2}}{n - 1}}$(The standard deviation of the R_CAPAE measurements, if n=1 thenSD_CAPAE=0)

${A\; V\;\_\; C\; A\; P\; B\; E} = {\sum\limits_{i = 1}^{n}\frac{\left( {R\;\_\; C\; A\; P\; B\; E} \right)_{i}}{n}}$(The average of the R_CAPBE measurements)

${S\; D\;\_\; C\; A\; P\; B\; E} = \sqrt{\sum\limits_{i = 1}^{n}\frac{\left. \left( \left( {\left( {R\;\_\; C\; A\; P\; B\; E} \right)_{i} - {A\; V\;\_\; C\; A\; P\; B\; E}} \right) \right) \right)^{2}}{n - 1}}$(The standard deviation of the R_CAPBE measurements, if n=1 thenSD_CAPBE=0)

To see if the remaining lines are “balanced”, we assess:If AV_CAPAE>P1_(—)11×AV_CAPBEAND AV_CAPBE>P1_(—)11×AV_CAPAE

If this condition is satisfied, then the line is balanced. For thisembodiment with the suggested values used, the balance condition wassuch that the average A and B leg to earth capacitance value are within80% of each other (i.e. average A leg to earth >0.8×average B leg toearth and average A leg to earth <1.25×average B leg to earth).Accordingly, as discussed above, a single representative average valueis obtained.

Lines that are not balanced are not included in the calculationsfurther.

To complete the assessment, the following is calculated:If SD_CAPAE<P1_(—)12AND SD_CAPBE<P1_(—)12

If this condition is fulfilled, then the line is considered to be“steady” as the deviation is within a preset range. Lines that are notsteady are similarly not considered further.

For balanced and steady lines, a healthy reference for the line is givenby

$H = \frac{{A\; V\;\_\; C\; A\; P\; A\; E} + {A\; V\;\_\; C\; A\; P\; B\; E}}{2}$

Having established a value of H for each line that has not beendisregarded for one of reasons set out above, a histogram of the typeshown in FIGS. 2A and 2B is compiled. FIGS. 2A and 2B shown histogramsfor two different PCPs, highlighting the significant difference thatoccur as physical conditions alter. The histogram values are entered in“bins” of a predetermined width, the height of the bin indicating thenumber of values within the range defined by bin width. For example,with a bin width of 5nf, a bin spanning 125 to 130nf and having a valueof 10 indicates that 10 lines registered capacitance values in thatrange.

Table 5 lists user definable parameters and suggested values for thepreparation of the histogram.

TABLE 5 Parameter Parameter Possible range Suggested Code DescriptionUnits to set parameter value P2_1 Histogram bin width Nano Greater than0 5 Farads P2_2 First run, number of — 3 or more 5 consecutively filledbins

For each PCP being considered, for all the lines that are routed throughthe PCP that have an associated H value, the following is calculated.Min_Health=minimum (H₁H₂ . . . H_(N))Max_Health=maximum (H₁H₂ . . . H_(N))where N is the number of lines routed through the PCP underinvestigation that have an associated H value.

Then, Min_Bin and Max_Bin are calculated, where

${Min\_ Bin} = {\left\lbrack {t\; r\; u\; n\; c\; a\; t\;{e\left\lbrack \frac{Min\_ Health}{{P2\_}\; 1} \right\rbrack}} \right\rbrack \times {P2\_}1}$

${Max\_ Bin} = {\left\lbrack {{t\; r\; u\; n\; c\; a\; t\;{e\left\lbrack \frac{Max\_ Health}{{P2\_}\; 1} \right\rbrack}} + 1} \right\rbrack \times {P2\_}1}$

${No\_ Bins} = \frac{\left( {{Max\_ Bin} - {Min\_ Bin}} \right)}{{P2\_}1}$

The N values of H for the lines routed through the PCP are sorted into anumber of bins. A bin is a count of the number of lines with H values inthe range defined by

-   H>=lower_range for the bin-   AND H<upper_range for the bin

Hence, each bin has an associated upper and lower capacitance value.

The lower_range and upper_range values for the bins dependent on the binwidth P2_1 are given by the following expressionslower_range_(a)=Min_Bin+a×(P2_(—)1)upper_range_(a)=Min_Bin+((a+1)×P2_(—)1)where a is an integer from 0 to (No_Bins−1)

The number of lines, count_lines_(a) in each of the No_Bins bins, isgiven by the expressioncount_lines_(a)=number of lines with H>=lower_range_(a)AND H<upper_range_(a)

This creates a histogram of the H values for the lines routed throughthe PCP.

Examples of two histograms for PCP's in one area are shown in FIGS. 2Aand 2B. The histograms have a 5 nf bin width, with individual bin startand stop values being multiples of five and the range of bin valuesbeing such that all the healthy reference values for the lines routedthrough the PCP's fit into a bin. As discussed above, it is not possibleto proceed to estimate capacitance values for the local switch to PCPmerely from the minimum average healthy leg to earth capacitance valuesfor all the lines routed through the PCP. This is because outlying, lowcapacitance values arise. Some of these are due to unknown DACS lines,others are due to lines re-routed or re-allocated since the creation ofrouting tables used in the analysis.

The outlying points are removed by finding the minimum of a distributionof the healthy reference values for the PCP from the histograms.

It is then necessary to identify the reference capacitance value for theswitch to PCP line, by identifying the lowest valid value. In thepreferred embodiment, to assess the valid minimum of the distribution,the lowest value bin is identified, scanning upwards from 0 nf, that hasat least P2_2 adjacent bins that contain data. The bin value lowest inthis range is taken as the switch to PCP capacitance reference. If nosuch range of adjacent bins is found, the process must be repeatedlooking for (P2_2)-1 adjacent bins.

For example, in FIG. 2A, the minimum distribution occurs at 120 nf.Lower values are not part of the main distribution, and are attributableto DACS and are included to illustrate the point that DACS lines arenoticeably different to other lines. In FIG. 2B, the minimum of thedistribution occurs at 60 nf.

The method for obtaining the minimum of the distribution of validhealthy references for PCP, P_(min) is shown in FIG. 3. Basically,P_(min) is the numerically lowest lower range value from the numericallylowest set of P2_2 or (P2_2)-1 consecutively filled bins, (i.e.count_lines_(a) not zero). In a preferred development, P_(min) can befurther selected as the lowest capacitance value within the selectedbin.

At step 100 a value of run=1 is set. At step 102 the run value is testedand if still 1 the process proceeds to step 104 where the number ofconsecutive bins no_consec_bins is set to the predetermined value P2_2,for example 5. A further check at 106 is carried out on run and as thevalue is less than 2 step 108 includes further initializing steps withthe following values:

-   sequence=0-   set start_lower_range=0-   set a=0

At step 110, if the value of count_lines_(a) is 0 (that is, the numberof lines in a given bin) then at step 112 a (the bin number) isincremented by 1 and sequence and start_lower_range are again set to 0.The process returns to step 110 to establish whether there are anyentries in the next bin a+1. The procedure continues until a bin isfound with entries in it and at step 114 sequence is implemented by 1.At step 116, if sequence=1 then start_lower_range is set to lower_rangea, that is, a first bin value at step 118. On the other hand, ifsequence is not 1 then at step 120 the sequence value is tested againstno_consec_bins (a predetermined lower limit for the number of adjacentbins).

If the values tally then the bin is identified as step 122 as lowerrange a. Otherwise, at step 124 the bin number is checked to see if ithas reached the upper bin number, a=(no_bins−1). If not, then at step127 the value of a is incremented and steps 110 to 120 are repeated forthe next bin until P2_2 of adjacent populated bins are identified.

If, at step 124, the value of a has reached the upper limit, then thevalue of run is incremented by 1 and the process returns to step 102. Asrun does not equal 1 then at step 128, the number of adjacent bins isdecremented by 1, no_consec_bins=(P2_2)-1. The process then moves tostep 106 and such 106 to 126 are followed once again. If (P2_2)-1consecutive filled bins are not found (the maximum number having beendecremented by 1) then the process returns to 102 once again, but atstep 106 the test “is run greater than 2?” is failed and the processstops at 130 as not enough consecutive filled bins can be found.

It will be appreciated that the determination of P_(min) is preferablybased on a plurality of values for each line taken over thepredetermined period, for example, on healthy lines overnight. In thatcase, the value can be continually updated to account, for example, forrouting changes or other variations. Alternatively, the healthy valuescan be taken from historical data. Yet further alternatively, thedetermination can be based on a single measured value for each line witha corresponding reduction in accuracy and foregoing various of thechecks for “healthy” reference values.

The reference value that is stored is used in the second part of theoperation for assessing the location of a fault as discussed in moredetail below. It should be borne in mind that a PCP capacitancereference value obtained as discussed above does not represent thecapacitance of a line disconnected at the PCP, but of the healthy linesclosest to the PCP. Therefore, the PCP reference is an over-estimate ofthe capacitance of a line disconnected at the PCP. This over-estimationis corrected for by the method of using the distribution of all historicdisconnection faults to diagnose future connection faults as explainedin more detail below. The convention of using the reference P_(min) forthe PCP reference obtained from the minimum of the distribution ofhealthy references will be maintained throughout the description.

The fault location algorithm used in the present invention is created byassuming that the line test characteristics of a set of historicdisconnection faults in a particular area can be used to diagnose thefinal clear-code (the code identifying the fault, given by an engineerdealing with the fault) of faults that occur subsequently in that area.This diagnosis is performed by examining the line test results of thenew faults and trying to match them to the distribution observed fromthe historic disconnection fault measurements.

The information used as input to the selection algorithm is presenteddiagrammatically in FIGS. 4 a and 4 b for the cases of a PCP fault and anon-PCP fault, in this example a drop wire fault, at the distributionpoint (DP) 145 respectively. For a particular fault, the differencebetween the healthy reference capacitance 140 and the fault capacitance142, 146 for the disconnected leg gives an idea how far the fault isfrom the customer 144. For PCP faults 142, in many but not all cases,the fault will be far (at least several nf) from the customer 144. As isshown in FIG. 4 a, PCP faults will certainly be further from thecustomer 144 than a drop wire fault or, for that matter, other faults onthe d-side of the PCP (cabinet) 146. The differences between the PCPminimum reference P_(min), 148 and the fault reference 142, 146 give anidea how close the fault is to the PCP 146. For many PCP faults 142 thisvalue will be small, provided the e-side cable length of the cable thedisconnection fault is on is close to the minimum e-side cable length(whose length is estimated using the minimum of the healthy linesdistribution P_(min)). In the example shown in FIGS. 4 a and 4 b, it isclear that the drop wire fault 146 has a capacitance measurement far inexcess of the PCP reference 148, whereas the PCP fault measurement 142is similar to the PCP reference 148. The difference between thecapacitance value of the disconnected leg for historic PCP faults,P_(hist) (as determined by faults historically assigned the appropriatefault code), and the fault reference for faults routed through the samePCP 146 also gives an idea how close the fault is to the PCP. Asdiscussed in more detail below this further comparison provides theadditional potential for identifying PCP faults 142 occurring on cablesother than the shortest e-side cable.

Accordingly, if a new disconnection fault has associated references andmeasurements that are similar to those of previous PCP disconnectionfaults and dissimilar to those of previous disconnection faults notcleared at the PCP (i.e. identified as occurring elsewhere), then it isprobable that the new fault is a PCP fault. In the preferred embodiment,the PCP and non-PCP faults are grouped in such a way that theprobability of a fault being a PCP fault could be calculated asdiscussed in more detail below. These probabilities, combined with somesimple threshold information, provide the basis for the selectionalgorithm.

In the preferred embodiment, in order to establish the probability of aparticular fault being a PCP fault tables are created. Examples of theseare shown in FIG. 5. As discussed in some detail above, a healthy valueH for each line is determined and a value P_(min) is also establishedrepresenting the PCT reference value. Historical disconnection faults inthe area are then reviewed to obtain values of F the fault capacitancefrom the switch/exchange 150 in FIGS 4 a and 4 b to the fault (faultmeasurement 142, 146). As a simple example, for a given line where:H−F=5 nf; andP _(min) −F=−45 nfthe implication is that the fault is not at the PCP but close to thecustomer. On the other hand where:H−F=52 nf; andP _(min) −F=2 nfthen the fault is likely to be close to or at the PCP and distanced fromthe customer. As a general rule H−F will generally take a large value ifthe fault is at the PCP whereas F will be a long way from P_(min) if thefault is non-PCP.

According to the invention, the historical data for F is compared foreach line according to these two rules and the table of FIG. 5 a showsthe results sorted into bins with (P_(min)−F) on one axis 160 and (H−F)on the other axis 162. Sample A shows a disconnection close to thecustomer whilst reference point B shows a disconnection close to thePCP. Multiple values are presented on the table and sorted into bins.

Carrying out this information for a large number of faults provides atable in which predictions can be made in relation to new faults. As newfaults arrive their co-ordinates are determined and mapped onto thetable of FIG. 5 a. From this the probability of the fault being a PCPfault can be determined based on the clusters previously identified. Asa result, based on pre-determined probability parameters, a decision canbe made as to whether the fault occurs at the PCP or distant from thePCP. This reduces the likelihood of the engineer being dispatched to thewrong location, rendering the maintenance operation more efficient. Ifwill be appreciated that separate tables may be needed for results fromexchanges in similar physical/geographical areas as the physicaldistribution of PCPs and customers can vary considerably between, forexample, urban areas and rural areas.

Preferably a second table is also compiled as shown in FIG. 5 b. Inorder to compile this second table a further value P_(hist) is required.As discussed above, P_(hist) is a historical value of the PCPcapacitance (for example taken from a line known to be disconnected atthe PCP). It will be appreciated that a range of such value areavailable, for example, because of differences in routing or temporalchanges. The value of P_(hist) closest to F is in fact selected and theaxes thus comprise abs (P_(hist)−F) at 164 and, once again, (H−F) at 166in FIG. 5 b. Selecting the value of P_(hist) closest to the fault valueF is based on the assumption that the fault is either on the same e-sidecable as that for which P_(hist) was measured, or a cable of similarlength.

Multiple values are shown plotted on the table of FIG. 5 b, allowing acluster around results similar to those of type a and b respectively inFIG. 5 a. The table of FIG. 5 a is referred to as a PCP reference tableand the table of FIG. 5 b is referred to as a historic reference table.The manner in which bins are determined and the probability of thenature of the fault is established and is discussed in more detailbelow.

These tables provide a method where the distribution of all disconnectedfaults and PCP disconnection faults can be examined. Also, the use ofsuch tables ensures that the location algorithm (discussed below) isbest suited to the area in which the trial is to be conducted (as theinformation used to build up the table is from that area). The methodalso side steps the problem that the P_(min) is an overestimate of theactual exchange to PCP reference for the shortest e-side cable feedingthe cabinet. This is because the table concerned is populated in such away that the offset introduced by the overestimation simply shifts thedistribution by some value along the P_(min)−F axis. However, it shouldbe noted that the amount by which the minimum of the distribution ofhealthy references overestimates the exchange to PCP capacitancereference will vary from PCP to PCP in the present invention is assumedthat this variation is negligible.

Turning now to the construction of the tables and their dimensions, thePCP reference table (FIG. 5 a) is created such that the (H−F)capacitance range covers all the values that might be formed. As aresult, the lowest bin on this “axis” of the table range from −2000 nfto −10.5 nf, where −2000 nf is an arbitrary large negative number. Thetable then has “useful” bins on this axis that are 1 nf wide whose lowerrange values increment by 1 nf from −10.5 nf to 79.5 nf. The final binrange in the healthy reference minus fault measurement capacitancedirection extends from 80.5 nf to 2000 nf, where 2000 nf is anotherarbitrary large number. The data bins into which the disconnectionfaults are sorted are formed by also subdividing the P_(min)−Fcapacitance range. In this case the lowest value bin extends from −2000nf to −50.5 nf, where −2000 nf is again an arbitrary large negativenumber. The table then has “useful” bins 1 nf wide whose lower rangevalues increments by 1 nf from −50.5 nf to 49.5 nf. The final range inthe P_(min)−F capacitance directed extends from 50.5 nf to 2000 nf,again 200 nf being an arbitrary figure. Hence a series of data bins iscreated, the “outer” bins being very wide, the “inner” bins being 1 nfwide in the H−F direction and 1 nf wide in the P_(min)−F direction. Thevast majority of faults would be sorted into the “inner” bins.

The historic PCP reference table (FIG. 5 b) uses the same (H−F) rangeand divisions as used in the PCP reference table. The (P_(hist)−F)capacitance range is 0 nf to 0.5 nf. The table then has (P_(hist)−F)capacitance bins of 1 nf width whose lower ranges increment in steps of1 nf from 0.5 nf to 49 nf. The last bin in this direction ranges from50.5 nf to 2000 nf, where 2000 nf is a large arbitrary value. The binranges are selected so that the majority of faults are in bins that were1 nf wide, whilst keeping the overall table size as small as possible.The algorithm to compile these tables are discussed subsequently inrelation to Table 10 from which it will be seen that the various valuesdiscussed above for example maximum and minimum bin values, bin widthand so forth are individually configurable by associating desired valueswith each available parameter. It will be appreciated that throughoutthe specification the values given for the tables are exemplary only andcan be replaced by any other appropriate values.

Each of the tables created is basically an array upon which any faultcan be placed according to its electrical characteristics (the historicreference table may not include all faults as not every faultnecessarily has a previous PCP fault on the PCP through which the linewas routed). Each bin in the tables preferably has two fields associatedwith it, one to hold the total number of faults with the electricalcharacteristics in the range of the particular table bin, the other tohold the number of these total faults that were cleared (i.e. identifiedat) the PCP. It should be noted that faults that are duplicate reportsor “faults-not-found” are not used in populating the probability tables(as in the latter case as no information as to the position of the faultin the access network can be extracted from such clear-codeinformation). The method used to populate the two probability tables isdetailed in the flow charts shown in FIGS. 6 a and 6 b.

Referring firstly to FIG. 6 a, at step 170 all faults are selected and aloop for each fault is set up at step 172. At step 174, if the fault isa disconnection fault with a healthy reference value H and PCP referenceP_(min) then at step 176 the difference (H−F) is calculated, at step 178the difference (P_(min—)−F) is calculated and at step 180 the relevantbin in the PCP reference table into which the fault falls is determined.At step 182 if the fault is a PCP fault (as determined by the engineerat the time of monitoring the fault and indicated by the fault code)then, in the selected bin, both the fault number and the PCP faultnumber is incremented by one at step 184. Otherwise the fault numberalone is incremented at step 186. Decision box 188 repeats the loopuntil no more cleared faults require examining at which point tablefilling is completed at step 190. Accordingly a PCP reference table isformed in which each bin includes the valued both total faults andrecorded PCP faults.

Turning now to FIG. 6 b, a similar series of steps is followed for ahistoric PCP reference table. At step 200 all faults are selected and aloop for each fault is instituted at step 202. At step 204 it isestablished whether the fault is a disconnection fault or is a value Hand also whether any PCP faults have occurred on the relevant PCP (asotherwise a value P_(hist) cannot be determined). If these conditionsare met that at step 206 (P_(hist)−F) and (H−F) are calculated and therelevant bin on the table is determined at step 208. At step 210, if thefault is a PCP fault then the both the fault number and PCP fault numberare incremented in the bin by one at step 212. Otherwise only the faultnumber is incremented at step 214. The loop is repeated at step 216 andonce all cleared faults have been examined table filling is complete atstep 218.

The pair of fields in each bin is provided in order to allow calculationof the probability of a given fault being a PCP fault. In particular ifa detected fault falls in a given bin then the probability of that faultbeing a PCP fault can be determined from the historical data in that binand in particular by dividing the number of historic PCP faults in thebin by the total number of faults in the bin. However, this is based onthe hypothetical position that each bin contains a representative sampleof faults but in practice some table bins may include only a very smallnumber of faults as a result of which the probability calculation maynot be accurate.

For instance in one bin there may be only two PCP faults, whereas in anadjacent bin there may be five faults, of which only one was cleared in(i.e. identified at) the PCP. It is unlikely that the probability of afault being a PCP fault would change dramatically from one bin to anadjacent bin, hence the source of large differences in probability frombin to bin lays in the low number of faults being considered.

In such instances a threshold value for the number of faults is requiredfor the probability obtained from the table. In order to obtain therequired number, the bins on the tables are “expanded” in the H−Fdirection. This is done by

Examining the initial table bin to see if a total of a predeterminednumber, for example 10 faults were present. If there are less than thepredetermined number of faults present examining the two adjacent binsin the H−F direction. Repeating the procedure of examining the next twoadjacent bins was repeated until either 10 faults are present or theentire range of H−F had been examined.

The sum of the PCP faults over the range of bins considered divided bythe sum of the total faults over the same range is then taken as theprobability of the faults being a PCP fault. The reason why the bins areonly extended in H−F direction is that the separation between the faultmeasurement and PCP reference is more important in deciding if a faultis at the PCP than how far the fault is back from the customer,irrespective of the method used to assess the PCP reference.

The flow chart in FIG. 7 provides a diagrammatic representation of themethod used to calculate the probability from the PCP reference table.The method used to get the probability from the historic PCP table isbroadly similar and will not be repeated here as it will be entirelyapparent to the skilled person.

At step 230 the process is commenced for each fault. At step 232, if afault is a disconnection fault with values H and P_(min) then at step234 (H−F) and (P_(min)−F) are calculated to determine the relevant binon the PCP reference table. At step 236 the bin width is initially setto 0 and a loop set up for each fault at step 238. At 240 the totalnumber of faults in the determined bin is compared to the pre-determinedvalue, for example 10 (as the bin width is set to 0, the bins eitherside are not assessed). If the number exceeds the predetermined valuethen at step 242 the total number of faults and total number of PCPfaults are summed allowing the probability determination to be carriedout by simple division. If in step 240 the total fault number does notexceed the predetermined value then, unless the entire range of H−F hasbeen covered, the bin width is incremented by one at step 244 and theassessment at step 240 is repeated until the total number of faultsexceeds the predetermined value. As a result a representative sample onthe basis of which a probability calculation can be completed isobtained by incrementing the bin width until sufficient total faults arecaptured. The operation is terminated at step 244 if a probabilitydetermination can be made or if the basic values cannot be ascertainedat step 232. Again, the various values discussed above are individuallyconfigurable by setting respective parameter values within thecontrolling algorithm as discussed below in relation to Table 11.

The selection algorithm is developed using the same set of faults aswere used to build up the PCP reference and the historic PCP tablestogether with a live fault measurement value, the relevant bin for whichis determined by its H−F and P−F values. The PCP fault probabilities areextracted from the tables as described above, however, the “live” faultitself is preferably removed from the probabilities by subtracting onefrom the total number of faults and if the fault is a PCP fault, thenumber of PCP faults is also decremented by one. This method ensuresthat the results forming the tables for each individual live fault arenot influenced by the fault itself and allows a large set of faults tobe used in establishing the form of the selection algorithm. In thepreferred example the algorithm is developed to ensure that as many PCPfaults are acted on as possible whilst ensuring that the accuracy(defined as the number of PCP faults acted on divided by the totalnumber of faults acted on) does not fall below some selected accuracy,for example 70%.

When a fault is examined (i.e. a “live fault”), the characteristics ofthe fault (the healthy reference, the fault measurement, the PCPreference and the historic PCP fault measurement closest to the faultmeasurement) are recorded along with the associated fault number. Aswell as the characteristics, the number of PCP faults, total faults andnumber of bins examined for each of the probability tables are alsorecorded for each fault (if there were no previous PCP faults then thefields associated with this parameter are left empty). The “selectionalgorithm” to decide if a fault is a PCP fault then examines both thefault probabilities and characteristics. This combination ofprobabilities and characteristics is required due to the problemsassociated with probabilities obtained when the bins spread over a largerange of H−F, which occur when faults fall in areas of the table wherethere were few historic faults. The examination of the faultcharacteristics provides a method whereby the accuracy of theprobabilities obtained for a particular fault could be assessed. Forexample, a fault may have occurred very close to the customer (lowchance of being PCP fault), but due to a low number of faults in thatpart of the table, the probability returned is calculated from manyfaults that had occurred far from their associated customers (that havea greater chance of being PCP faults). In this case, the “high”probability of a fault being a PCP fault can be ignored on the strengthof the argument that a fault very close to a customer is unlikely to bea PCP fault.

The selection algorithm for determining probability is composed of twosections. The first examines the probability from the PCP referencetable (FIG. 5 a), requiring in the preferred example that theprobability be greater than some selected probability, for example 70%for the fault to be acted on. The second examines the combinedprobability from the PCP reference (FIG. 5 a) and the historic PCP (FIG.5B) tables, requiring that the combined probability be greater than someselected probability, for example 70%. As a result inaccuracies in thehistoric PCP table preferably arising because the e-side cable for theselected historic PCP fault P_(hist) was not always on the same e-sidecable as the fault itself are investigated. Because of theseinaccuracies the historic PCP faults had, in some cases, characteristicssimilar to non-PCP faults on shorter e-side cables.

According to the algorithm, and dependent on the PCP selected, there arevarious thresholds that must be adhered to in order for a fault to beacted upon.

-   (a) The total number of faults in the expanded bin must exceed one.    Ideally there should be at least some predetermined number of faults    when calculating the probability as discussed above. However, where    there is a small number of faults in parts of the table, the    situation could arise where there was only one fault, which is a PCP    fault, in the H−F column in either the PCP reference or historic    reference tables. If such cases were considered then all future    faults with the same H−F value would be diagnosed as PCP faults    (until the table is re-populated). Increasing this value for the    number of faults being considered when calculating the probability    significantly reduces the number of faults acted on.-   (b) The fault measurement must be less than the healthy measurement.-   (c) F must be greater than or equal to, for example, 0.6×P_(min), to    remove exchange faults.-   (d) F must be less than or equal to, for example, 1.75×P_(min), to    remove the influence of incorrectly cleared faults and PCP faults    for lines on e-side cables other than the e-side cable through which    the fault being tested was routed.

Having looked at the general operation of this aspect of the inventionwe now consider in more detail the algorithm's operation involved incompiling the histogram and assessing the relevant probabilities.

Table 7 defines a set of user definable parameters and suggested valuesfor the preparation of historical fault data as part of the algorithms.

Possible Parameter Parameter range to Suggested Code description Unitsset parameter value P3_1 Minimum number — 10 or more 10 or more of weeksof fault data required P3_2 Minim um fault Ohms 0 to 999999 P1_4resistance threshold for A leg to Earth P3_3 Minimum fault Ohms 0 to999999 P1_7 resistance threshold for B leg to Earth P3_4 Minimum faultOhms 0 to 999999 P1_3 resistance threshold for A leg to B leg P3_5Minimum fault Ohms 0 to 999999 P1_8 resistance threshold for B leg tobattery P3_6 Minimum fault Ohms 0 to 999999 P1_6 resistance thresholdfor B leg to A leg P3_7 Minimum fault Ohms 0 to 999999 P1_5 resistancethreshold for A leg to battery P3_8 Disconnection fault, — Greater thanIF Vanderhoff loop upper threshold 0 to 1 THEN value P3_8 = 0.5 IFTeradyne THEN P3_8 = 1.0 P3_9 Disconnection fault, — 0 to 1 IFVanderhoff loop lower threshold THEN value P3_9 = 0.0 IF Teradyne THENP3_9 = 0.0

Table 8 provides the codes and descriptions for fault information forthe area being investigated.

Code Name Description F_FN FAULT_NUMBER CSS fault reference code F_CIRCCIRCUIT_ID Identifies the line F_CCODE CLEAR_CODE CSS clear code) note,this is not a number)

Table 9 sets out the relevant codes and descriptions for associated linetest information of each fault.

Code Name Description T_FN FAULT_NUMBER CSS fault reference code T_CIRCCIRCUIT_ID Identifies the line T_SEQ SEQUENCE_NUMBER Number of the linetest done T_RESAE RES_A_ETH Resistance value, used to check line T_RESBERES_B_ETH Resistance value, used to check line T_RESAB RES_A_BResistance value, used to check line T_RESBBAT RES_B_BATT Resistancevalue, used to check line T_RESBA RES_B_A Resistance value, used tocheck line T_RESABAT RES_A_BATT Resistance value, used to check lineT_TERM LINE_TERM_FOUND Test system response, Y or N T_CAPBE CAP_B_ETHCapacitance, B leg to Earth T_CAPAE CAP_A_ETH Capacitance, A leg toEarth T_CAPLOOP CAP_B_A Capacitance, loop

Tables 8 and 9 are based on conventionally used data fields.

Fault data with associated line test information is obtained for a timeperiod defined by P3_1. The period is dependent on a number of linesbeing considered in order to ensure that a suitable population of datais obtained. For example for 100,000 lines a 13 week period may beappropriate. For each fault the directory number is obtained fromCIRCUIT_ID. A particular embodiment of the system for locating faults isdependent on the system used for testing, if healthy data is availablefor only one circuit on a directory number, then all the circuits onthat are assumed to have the same H value. As a result any fault on amulti-line installation uses the H value for the circuit for which datais available. If additional routine data is available then the systemcan accommodate this, for example, by creating H values for each circuitand comparing fault data for that circuit with the associated H value.

It will be noted that any faults on lines having an associated H valueand a P_(min) value for the relevant PCP are considered. In the presentsystem measurements are examined where T_SEQ=1 such that only the firsttest on the fault is considered to obtain a single set of measurementsfor the fault (an average of all the test measurements for the faultcould possible be used). For multi-line installation it is assumed thatthe first test is on the circuit on which the fault has been reported.

Various further requirements are introduced to restrict the faults thatare considered. Once again the tests are conducted on lines which do nothave any resistance values less than some predetermined value, forexample 1 MΩ. As can be seen the various possible resistance values foreach leg and between the legs and earth are all checked. Of these lines,disconnection faults are taken as being those with a loop capacitancevalue less than or equal to P3_8×minimum of the A or B leg to earthcapacitance values and greater than or equal to P3_9×minimum A or B legto earth capacitance (the value of P3_8 is dependent on the test system;where the Vanderhof test systems used the value of P3_8 is 0.5) andwhere the line system has not detected a termination. This isdemonstrated by the following conditions:

-   -   T_RESAE>P3_2    -   AND T_RESBE>P3_3    -   AND T_RESAB>P3_4    -   AND T_RESABAT>P3_5    -   AND T_RESAB>P3_6    -   AND T_RESABAT>P3_7    -   AND    -   (T_CAPLOOP<=P3_8×Minimum (T_CAPAE,T_CAPBE)    -   AND T_CAPLOOP>=P3_9×Minimum (T_CAPAE,T_CAPBE)    -   AND T_TERM=N).

The fault measurement is taken as the minimum of the A or B leg to earthcapacitance value. As a result, and as discussed above, only balancedlines are considered to assist in the selection of the minimum value.Only faults upon balanced lines are considered as the method of takingthe minimum leg to earth capacitance value for the fault measurementwould not work if the line was severely imbalanced (as the leg with theminimum capacitance value may not be the leg upon which disconnectionhas occurred). Once again, the fault measurement is referred to as F,and can be determined as follows:

-   F=Minimum (T_CAPAE,T_CAPBE).

The fault is also disregarded if F_CCODE is one of various predeterminedcodes indicating exceptional circumstances beyond the scope of thesystem. Any remaining faults are labelled as either a PCP or a non-PCPfault, the fault code indicating any PCP fault.

A list of historic PCP faults is thus compiled where the PCP historicfault reference P_(ref) is taken as F, ieP_(ref)=F.

Accordingly a reference value is stored for PCP faults on each line andin addition routing, R information for the associated lines is alsostored. All remaining faults are non-PCP faults, by definition,

Turning now to the probability table that is compiled, table 10 showsuser definable parameters and suggested values for the probability tablefill in. These are domain parameters, each region having an individualset of such parameters.

Parameter Parameter Possible range Suggested code description Units toset parameter value P4_1 PCP reference table Nano Minus infinity to−2000 H-F start value Farads less than P4_2 P4_2 PCP reference tableNano Greater than −10.5 H-F lower “bin Farads P4_1 width” range valueP4_3 PCP reference table Nano (Greater than 0) 1 H-F bin width valueFarads to 10 P4_4 PCP reference table Nano P4_2 plus some 80.5 H-Fhigher “bin Farads positive, non- width” range value zero, integermultiple of P4_3 P4_5 PCP reference table Nano (Greater than 2000 H-Fend value Farads P4_4) to infinity P4_6 PCP reference table Nano Minusinfinity to −2000 Pmin-F start value Farads less than P4_7 P4_7 PCPreference table Nano Greater than 4_6 −50.5 Pmin-F lower “bin Faradswidth” range value P4_8 PCP reference table Nano (Greater than 0) 1Pmin-F bin width Farads to 10 value P4_9 PCP reference table Nano P4_7plus some 50.5 Pmin-F higher “bin Farads positive, non- width” rangevalue zero, integer multiple of P4_8 P4_10 PCP reference table Nano(Greater than 2000 Pmin-F end value Farads P4_9) to infinity P4_11 PCPhistoric fault Nano Minus infinity to −2000 table H-F start Farads lessthan P4-12 value P4_12 PCP historic fault Nano Greater than −10.5 tableH-F lower Farads P4_11 “bin width” range value P4_13 PCP historic faultNano (Greater than 0) 1 table H-F bin width Farads to 10 value P4_14 PCPhistoric fault Nano P4_12 plus some 80.5 table H-F higher Faradspositive, non- “bin width” range zero, integer value multiple of P4_13P4_15 PCP historic fault Nano (Greater than 2000 table H-F end valueFarads P4_14) to infinity P4_16 PCP historic fault Nano Greater than 00.5 table Rhist lower Farads “bin width” range value P4_17 PCP historicfault Nano (Greater than 0) 1 table Rhist bin Farads to 10 width valueP4_18 PCP historic fault Nano P4_16 plus some 50.5 table Rhist higherFarads positive, non- “bin width” range zero, integer value multiple ofP4_17 P4_19 PCP historic fault Nano (Greater than 2000 table Rhist endFarads P4_18) to value infinity

As discussed in more detail below tables are constructed allowing theprobability of a fault being a PCP fault to be estimated. Bearing inmind geographical/physical differences between networks in differentregions, several sets of tables may be required to accommodate this. Inaddition different line test system types (for example Vanderhoff orTeradyne) will give rise to different test results and also need to beaccommodated in different tables.

As discussed above, a PCP reference table (PR table) is created havingthe axes H−F and P_(min)−F. The H−F axis ranges are defined between twoouter limits P4_1 and P4_5. A “useful” bin range is defined by innerparameters P4_2 and P4_4, and the bin width within that range is definedby P4_3. This is expressed as follows:H−F>=P4_(—)1 to H−F<P4_(—)2H−F>=P4_(—)2+b(P4_(—)3) to H−F<P4_(—)2+(b+1)(P4_(—)3)H−F>=P4_(—)4 to H−F<P4_(—)5.where b is an integer ranging from 0 to (((P4_4_P4_2)/P4_3))−1).

In relation to the P_(min)−F axis, outer limits are defined by P4_6 andP4_10, inner, useful limits are defined by P4_7 and P4_9 and a usefulbin width is defined by P4_8, expressed as follows:P _(min) −F>=P4_(—)6 to P _(min) −F<P4_(—)7P _(min) −F>=P4_(—)7+c(P4_(—)8) to P_(min) −F<P4_(—)7+(c+1)(P4_(—)8)P _(min) −F>=P4_(—)9 to P _(min) −F<P4_(—)10.where c is an integer ranging from 0 to (((P4_9−D4_7)/P4_8))−1).

As discussed above two values are associated with each bin, the totalnumber of faults (PR_T) and the total number of PCP faults (PR_P). Theseare defined as follows:

-   PR_T=number of PCP faults and Non-PCP faults whose H−F and P_(min)−F    values are within the bin ranges    -   PR_P=number of PCP faults whose H−F and P_(min)−F values are        within the bin ranges.

The PR table is compiled from these values.

Turning now to the PCP historic reference table (PH), the axes asdiscussed above are (H−F) and R_(hist), are_(hist). R_(hist) is (basedon P_(hist)−F) in the following manner:R_(hist)=minimum_(r=l) ^(r=k)(absolute(Pref_(r)−F))where r is an integer and k is the number of Pref values that arepresent for the PCP associated with a fault being considered, excludingthe fault under consideration if that is a PCP fault. Note that R_(hist)can not be calculated if there has not been an historic PCP fault on thePCP associated with the fault. In this case the fault is not enteredinto the PH table.

The axis range for H−F has a lower limit P4_11 and an upper limit P4_15,inner useful limits P4_12 and P4_14 and a bin width in the useful rangedefined by P4_13, as expressed as follows:H−F>=P4_(—)11 to H−F<P4_(—)12H−F>=P4_(—)12+d(P4_(—)13) to H−F<P4_(—)12+d+1)(P4_(—)13)H−F>=P4_(—)14 to H−F<P4_(—)15where d is an integer ranging from 0 to (((P4_14−P4_12)/P4_13))−1).

The R_(hist) axis has a lower limit of 0 (as the value is an absolutevalue), and an upper limit defined by P4_19 in the useful limits definedby P4_16 and P4_18 and a bin width within this useful range defined byP4_17, as can be seen from the following:R _(hist>=)0 to R _(hist) <P4_(—)16R _(hist) >=P4_(—)16+e(P4_(—)17) to R _(hist) <P4_(—)16+(e+1)(P4_(—)17)R_(hist) >=P4_(—)18 to R_(hist) <P4_(—)19Where e is an integer ranging from 0 to (((P4_18−P4_16/P4_17))−1).

Once again each bin on the PH table has two fields, the total number offaults and the total number of PCP faults, defined as follows:

-   -   PH_T=number of PCP faults and Non-PCP faults whose (H−F) and        R_(hist) values are within the bin ranges    -   PH_P=number of PCP faults whose (H−F) and R_(hist) values are        within the bin ranges.

Turning to FIGS. 8 and 9 the manner in which the PR and PH tables arefilled or compiled respectively can be seen.

Dealing firstly with the PR table (PCP reference table), as shown inFIG. 8 at step 250 all faults are selected and a loop is commenced foreach fault at step 252. At step 254, if a line does not have H andP_(min) values then the process jumps to step 256 where the loop isrecommenced at 252 for the next fault unless no faults remain in whichcase at step 258 the process stops. If there are H and P_(min) valuesthen at step 256 a check is carried out to establish that the fault iseither a PCP or non-PCP fault (i.e. not one of the excluded faultsdiscussed above). If not then again the process jumps to step 260 andthe loop is repeated for any remaining faults. The process proceeds tostep 262 to establish that the fault is a disconnection fault, onceagain if not then the procedure jumps to the next fault at box 256.Otherwise F, H−F and P_(min)−F are calculated at box 264, 266 and 268respectively. The appropriate bin in the PR table is determined based onthe calculations set out above at step 270. At step 272, if the fault isa PCP fault then PR_T (total faults) and PR_−P (PCP faults) are eachincremented by one in the relevant field of the appropriate bin in thePR table at step 274. If the fault is not a PCP fault then only the PR_T(total faults) is incremented by one in the appropriate bin in the PRtable at step 276. The procedure then loops for any remaining faults asdetermined at box 256.

Compilation of the PH table is roughly equivalent and the steps aresimilarly numbered other than the addition of suffix a in each case. Theonly box that requires additional commentary are steps 254 a in whichcase there is an additional check for P_(ref) value, if not then noentry can be made and the procedure loops to the next fault at box 256a.

It should be noted that all the faults for the whole area underconsideration are used for the single PR and a single PH table.

We now turn to the “location algorithm” for locating a new or live faulttaking into account the use of definable parameters and suggested valuesfor the location algorithm set out in table 11 which one againrepresents “domain” parameters for a given region or embodiment.

TABLE 11 Parameter Parameter Possible range Suggested code descriptionUnits to set parameter value P5_1 Close to Customer Nano −10 to 20   2.5reference value Farads P5_2 Lower range of F — Greater than 0 to 0.6value used less than 1 P5_3 Upper range of F — Greater than 1 1.75 valueused P5_4 Minimum number — 1 or more 10 of faults summed over the rangeof bins P5_5 Minimum number — 1 or more 1 of faults obtained from the PRtable P5_6 Minimum — Greater than 0.7 probability of PCP 0 to 1 faultusing PR table only P5_7 Minimum number — 1 or more 1 of faults obtainedfrom the PH table P5_8 Minimum — Greater than 0.7 probability of PCP 0to 1 fault using PR and PH tables

When considering a new fault, first of all the ‘T’ values set out intable 9 are obtained and the fault is disregarded unless there is a Hvalue for the associated line, a P_(min) value for the relevant PCPthrough which the line is routed, and the fault is a disconnectionfault. F, (H−F), (P_(min)−F) and (R_(hist)) are all calculated asdescribed above, the last value again only being available if there hasbeen a historic PCP fault on the PCP through which the line is routed.The fault is ignored under various additional conditions, if it is tooclose to the H value (the threshold being defined by P5_1), or if it istoo small or large compared to P_(min) (defined by ratios P5_2 andP5_3), expressed as follows:IF (H−F<P5_(—)1OR F<P5_(—)2×P _(min)OR F>P5_(—)3×P _(min))THEN ignore the fault.

Otherwise the relevant bin is identified for the fault and theprobability of it being a PCP fault is calculated as discussed above,and is set out in more detail below in the following discussion. Inparticular the system is arranged to ensure that the probabilitycalculations are based on a suitable statistical population by summingacross a number of adjacent bins (identified on the H−F axis by integernumbers h_lower and h_upper in the negative and for the directionsrespectively) until the threshold population for the total number offaults to obtain a useful population P5_4 is reached or the entire rangeof bins has been examined. The two values for the total number of faults(sum_PR_T) and total number of PCP faults historically (sum_PR_P) areobtained from the following equations, as discussed with reference toFIG. 10:

${{SUM\_}\; P\; R\;\_\; T} = {\sum\limits_{g = {\_\; h\;\_\; l\; o\; w\; e\; r}}^{g = {{+ \; h}\;\_\; u\; p\; p\; e\; r}}{P\; R\;\_\; T_{g}}}$${{SUM\_}\; P\; R\;\_\; P} = {\sum\limits_{g = {\_\; h\;\_\; l\; o\; w\; e\; r}}^{g = {{+ h}\;\_\; u\; p\; p\; e\; r}}{P\; R\;\_\; P_{g}}}$where PR_T₀ is the PR_T value for the bin identified by the H−F andP_(min)−F values calculated for the fault.

Referring to FIG. 10, the process is commenced for each fault at step280 and, calculating H−F and P_(min)−F for each fault the relevant binof the PR table is determined at 282. At 284 h_lower and h_upper are setto zero and a loop for the fault is commenced at 286. At 288, on theinitial run h_lower=0 and hence will be less than or equal to the numberof bins on the H−F axis between the bin in which the fault falls and thelowest populated bin and the operation will proceed to step 242 wherethe value of h_upper will similarly be less than or equal to the numberof bins between the selected bin and the highest populated value bin.Accordingly at step 294 the total fault number SUM_PR_T is calculatedfor that bin and at 296, if that total is greater than the predeterminedbin value P5_4, such that a representative sample is obtained, then atstep 298 the PCP fault number SUM_PR_P is calculated and the calculationis completed for that fault after which the process can be looped againfor the next fault. If at step 296 the total fault number does notexceed the predetermined threshold P5_4 then at step 300, if there areno further bins to examine then the PCP fault number is calculatedanyway at step 298. Otherwise h_lower and h_upper are each incrementedby one. The checks at 288 and 292 set the value of h_lower and h_upperto a maximum value respectively when effectively one or other ends ofthe populated bins are reached as a result of which, at step 300, thecalculation will terminate.

A similar approach is followed to obtain the nominal values of the PHtable (where there has been an historical PCP fault on the relevant PCP)to obtain the equivalent total fault and PCP fault values:

${{SUM\_}\; P\; H\;\_\; T} = {\sum\limits_{i = {\_\; j\;\_\; l\; o\; w\; e\; r}}^{i = {+ {j\_ upper}}}{P\; H\;\_\; T_{i}}}$${{SUM\_}\; P\; H\;\_\; P} = {\sum\limits_{i = {{\_ j}{\_ lower}}}^{i = {+ {j\_ upperi}}}{P\; H\;\_\; P_{i}}}$

Once again the j_lower and j_upper values are incremented until thetotal number of faults exceeds 10.

From the four sum values the probability of a fault being a PCP fault isthen determined based on various initial requirements. Thoserequirements are that the total number of faults summed on the PR tableexceeds a minimum value P5_5 and the number of PCP faults summed on thePH table exceeds a proportion of the total summed faults, the proportionbeing determined by P5_6. Alternatively the total number of faultssummed on the PR table needs to exceed the base value P5_5 and the totalnumber of faults summed on the PH table needs to exceed a furtherthreshold defined by P5_7. In that case the ratio of PCP faults to totalnumber of faults in the relevant bin or bins is calculated for eachtable and compared against a threshold probability set at P5_8, forexample 70%. This is represented by the following expressions:IF(SUM_PR_T>P5_(—)5AND(SUM_PR_P>=P5_(—)6×SUM_PR_T_))OR(SUM_PR_T>P5_(—)5ANDSUM_PH_T>P5_(—)7AND((SUM_PR_T×SUM_PH_P)+(SUM_PH_T×SUM_PR_P)−(SUM_PR_P×SUM_PH_P)>=P5_(—)8×(SUM_PR_T×SUM_PH_T))).

It will be seen during the final calculation the threshold is calculatedbased on the combined possibility of both tables.

Of course, in cases where there are no historical PCP measurements thenthe algorithm will still work based purely on the PR table.

It is possible that if more faults were added to the two probabilitytables (i.e. the probability tables are updated when more fault databecomes available) that the values used in the selection algorithm wouldneed to be modified.

As an alternative to using historically collated data, for example,collated from faults logged in a ten week period, the table can beupdated in real time, adding new fault data to the table as it iscollected allowing the population of faults in the probability tables togrow over time. In such instance, for each fault various additionalinformation is preferably collected as follows:

-   Fault number-   Clear code (identifying fault type)-   Healthy reference from before the time the fault occurred−H-   Fault measurement−F-   PCP reference used when assessing the fault−P_(min)-   Routing information for the fault was assessed (zone, exchange, PCP,    PCP e-side termination).

The routing at the time of the fault would need to be recorded using thecoding described above as currently the fault number is referenced tothe phone number upon which the fault occurred and the routinginformation of this phone number is used as the routing for the fault.In an extended trial or operational system, the routing table would beupdated from time to time. As the phone number could be reassigned orthe routing altered in light of changes in the network, it is importantthat the routing of the fault is recorded, as the routing of theassociated line may change with time.

Should subsequent recalculations of healthy references on the lineoccur, and hence the PCP reference values calculated change, the valuesthat were employed at the time of the fault being assessed may be kept.The healthy reference has to be from the time before the fault wascleared in case the routing was altered when the fault was repaired orduring other subsequent changes. The PCP reference values may be updatedin the light of subsequent recalculation of this value. However,provided the method used to calculate the PCP reference remainsunchanged, then the PCP reference calculated at any time represents thebest value at that moment. Hence the PCP reference probability tablewould refer to the best estimates of the PCP reference at the moment thefault was assessed (as opposed to an “actual” PCP reference, which it isnot possible to determine exactly using P_(min), the minimum of thehealthy reference distribution method).

When a new fault is being assessed, the usual method of determining thefault location is employed.

When the fault is cleared it is then entered in the historic faultstable and the probability tables updated. If the clear code of the newfault is

“Not PCP”, then the probability tables could be updated simply by addingin the new fault.

“PCP” then the information stored for some of the historic faults mayhave to be altered as some faults may gain a P_(hist) value or theP_(hist) value may change. As a result, the historic referenceprobability table would have to be updated in light of this new recentlycleared fault and any changes for the existing historic faults. The PCPreference probability table could be updated simply by adding in the newfault.

In some areas each line is tested each night to obtain thecharacteristics of the line at that time so that an up to date healthyreference for each line in the trial can be obtained. The healthyreferences can be calculated from some predetermined number of recenthealthy line test measurements, for example, the last 10 healthymeasurements. This method ensures that almost all the lines in the areabeing tested have a healthy reference value and any changes in thereference value due to lines being re-routed can be captured. In thiscase, it may be necessary to recalculate the minimum of the healthyreference distribution on a periodic basis to reflect the changes thatoccur in the healthy reference values.

As more faults are logged it may become possible to build up thedistribution of disconnection faults on individual PCPs. Currently theselection algorithm works on tables that are essentially an average ofthe behavior of all the PCPs examined. The current method has the greatstrength that it allows disconnection faults on PCPs without previousPCP faults in the database to be assessed to see if the new fault is aPCP fault. However, the method does assume that the offset betweenP_(min) and the actual exchange to PCP capacitance reference is the samefor all PCPs. This does occasionally lead to some faults being diagnosedas PCP faults when they are on the d-side of the cab (such as faults inthe joints underground on the d-side). Individual PCP distributionswould allow the offset between P_(min) and the actual exchange to PCPcapacitance reference to be assessed for the particular PCP underexamination allowing more accurate locations to be diagnosed.

This invention produces a P_(min) an estimate of the minimum exchange toPCP capacitance reference for all the lines routed through the PCP.Alternatively the minimum exchange to PCP capacitance reference can becalculated for sets of lines running through the PCP. The lines aresorted into batches of 100 lines according to the e-side terminationnumber of the line employing this method, a reference for each e-sidecable feeding the PCP can be calculated.

Because the operation is based on comparison between P_(min), thehealthy line reference, and a measurement of the capacitance of a faultyline, there is no need to work back to the distance values or thecapacitance values because of the use of the capacitance valuesdirectly. Because the system is based on the PCP locality, the physicallocation of where the fault stems from is defined meaning that otherreference points in the network, be it at the exchange or the customer,are not required.

In the embodiment described the types of fault identified aredisconnection faults, that is lines where one or both of the legs havebeen broken at some point along their length. It will be appreciated,however, that alternative types of faults can be addressed. It will beappreciated that various additions and modifications of the embodimentsdescribed are contemplated. Although the discussion includes referenceto a minimum of a capacitance distribution, it will be appreciated thatthe invention can be extended to other parameter measurements from otherlimit values, such as maximums. Although the discussion is specificallyin relation to disconnection faults and to establishing whether a faultis at the PCP, the invention can be extended to other types of fault andother nodes. Although the data is effectively processed in histograms inthe embodiments discussed, alternative ways of presenting or processingthe data can of course be used. The particular axes implemented in thehistograms in the described embodiments are preferable as they allowcalculations to be made based purely on the capacitance values withoutconversion. In addition, by introduction of the P_(min) and P_(hist)values the histogram automatically compensates for the off-set caused bythe exchange to PCP capacitance. However, alternative or additional axescan be used based on the various values obtained and, for example, threeor more dimensional histograms can be used.

In the embodiments discussed, principally historical data can be used toform the various calculations or alternatively “live” data can becontinually added to the various values based on ongoing checks. In thatcase, the data population can be increased, and accuracy, and up-to-dateinformation dealing with, for example, routing changes can bemaintained. For example where a line is identified by a customertelephone number then changes to the telephone number can beaccommodated by such a system. The accuracy of the system can be setarbitrarily other than 70% probability as discussed above. Coverage (aproportion of faults picked up) can also be varied although increasingthis may increase the number of incorrect fault locations.

1. A line parameter estimation method for a telecommunications networkincluding a switch and a plurality of terminating lines extending fromthe switch through a plurality of nodes, said method comprising:measuring a line capacitance parameter of each terminating line;compiling line test data from the measured line capacitance parametersas a histogram where the minimum valid capacitance value is a minimumbin value that is the value of the lowest value bin in the lowest valueset of n consecutive populated bins, where n is a predetermined integer;calculating a valid limit value from the compiled line test data; andsetting a respective estimated switch to node line capacitance parameterfor each of the plurality of nodes as the valid limit value.
 2. A methodas claimed in claim 1 in which the valid limit value is a minimum validline capacitance parameter value.
 3. A method as claimed in claim 1 inwhich if n consecutive bins are not populated, n is decremented by
 1. 4.A method as claimed in claim 1 in which at least one of the nodes is aprimary cross-connection point.
 5. A method as in claim 1 wherein validcapacitance values exclude values which are substantially independent ofthe length of a line.
 6. A method as in claim 1 wherein the compiledline test data includes only healthy measured capacitance values.
 7. Amethod as claimed in claim 6 in which healthy values excludemeasurements from lines for which a corresponding measured resistanceassociated with the line is less than a predetermined value.
 8. A methodas claimed in claim 7 in which the predetermined value is 1 MΩ.
 9. Amethod as claimed in claim 7 in which the predetermined value is 100 KΩ.10. A method as claimed in claim 6 including the step of calculatingstandard deviation for each set of measured capacitance values, for aline, healthy values excluding measurements for lines for which thestandard deviation is greater than a predetermined amount.
 11. A methodas claimed in claim 6 in which the terminating lines each comprise firstand second legs and a value representative of the measured capacitanceof each leg is compared, in which healthy values exclude measurementsfrom lines where the difference between compared values exceeds apredetermined limit.
 12. A method as in claim 1 wherein the capacitancevalues are measured repeatedly for each line over a predetermined periodof time.
 13. A line parameter estimation system for a telecommunicationsnetwork including a switch and a plurality of terminating linesextending from the switch through a plurality of nodes, the systemcomprising: measuring means to measure at least a line capacitanceparameter of each terminating line; means to compile line test data fromthe measured capacitances as a histogram where the minimum validcapacitance value is a minimum bin value that is the value of the lowestvalue bin in the lowest value set of n consecutive populated bins, wheren is a predetermined integer; calculating means to calculate a validlimit value from compiled line test data; and estimating means todetermine a respective switch to node line parameter as the valid limitvalue for each of the nodes.
 14. A system as claimed in claim 13 inwhich the valid limit value is a minimum valid parameter value.
 15. Afault management system including a line parameter estimation system asclaimed in claim
 13. 16. A method for deriving an estimated validreference or normal switch-to-node line parameter value in atelecommunication network including a switch having a plurality oftelecommunication lines extending therefrom to remote terminationsthrough a plurality of intermediate network nodes, said estimated validswitch-to-node line parameter value being derived by: measuring a linecapacitance parameter value for each of said plurality of terminatinglines; compiling a histogram of said measured line capacitance parametervalues wherein the number of measured parameter values falling within abin range of values are accumulated for successively increasing binvalues; deriving a valid estimated reference switch-to-node linecapacitance parameter value from said compiled histogram of measuredline capacitance parameter values.
 17. A method as in claim 16 whereinsaid derived valid estimated reference value is used to locate theapproximate position of a fault in a telecommunications network.
 18. Amethod as in claim 1 wherein said estimated valid limit value is used tolocate the approximate position of a fault in a telecommunicationsnetwork.
 19. A line parameter estimation system as in claim 13 furthercomprising: fault location means for using the estimated valid limitvalues to locate the approximate position of a fault in atelecommunications network.